1. FIELD OF THE INVENTION
The present invention relates to the spray drying of liquids or slurries and more particularly, to an improved spray dryer discharge system that permits the immediate exit of dried material from the chamber of a spray dryer.
2. DESCRIPTION OF THE PRIOR ART
As known in the art, the term "drying" refers to, and may be broadly defined as, the removal of water from solids, liquids, or gases. The drying of solids may be effected in various apparatus including tray dryers, tunnel dryers, rotary dryers and the like. The drying or removal of water from gases may be effected, for example, by adsorption on a solid, such as silica gel or alumina. In general there are two basic apparatus employed for drying liquids or slurries. These are the drum dryer and the spray dryer. The former consists of one or more heated metal rolls on the outside of which a thin layer of liquid is evaporated to dryness. The dried solid is scrapped off the rolls as they slowly revolve. In a spray dryer a liquid (i.e., a solution) or slurry is dispersed into a stream of hot gas in a manner such that water is rapidly vaporized, leaving residual solid particulates. Many types of commercial dryers are known and described in standard references such as Riegel, E. R.: "Chemical Process Machinery", 2ed., Chapter 17, Reinhold Publishing Corp., New York, (1953) and Perry, J. H.: "Chemical Engineers Handbook", 4th ed., McGraw-Hill Book Co., Inc., New York (1960).
In recent years spray dryers have found increasing and ever expanding uses in chemical and related industries due to their very short drying time and the ability to obtain the desired consistency, bulk density, appearance, and flow properties of many products, such as foods or synthetic detergents.
As briefly discussed above, in a spray dryer a slurry or solution is dispersed into a stream of hot gas in the form of a mist of fine droplets. Moisture is rapidly vaporized from the droplets leaving residual particles of dry solid, which are then separated from the gas stream. The flow of liquid and gas may be cocurrent, countercurrent or a combination of both. The droplets are formed (usually within a generally vertical and cylindrical drying chamber) either by spray nozzles or by high-speed spray wheels or discs. Spray drying chambers having diameters on the order of 5 -30 feet are common in the industry.
In a cocurrent spray dryer, liquid feed is pumped or introduced into a spray nozzle or wheel positioned in the top or roof of the chamber. The spray assembly atomizes the liquid into tiny droplets which are thrown radially into a stream of hot gas, which enters near the top of the chamber. Cooled or exhaust gas is removed through a discharge line positioned in the vertical side walls of the chamber. The dry solids settle out of the gas into the bottom of the drying chamber and are conventionally withdrawn therefrom by a screw conveyor through a rotary airlock.
In a countercurrent unit, the liquid solution or slurry is introduced into the spray assembly (also positioned in the roof or upper zone of the drying chamber) with the stream of hot gas being introduced at a point near the bottom or intermediate of said drying chamber and in an upwardly direction i.e., "countercurrent" to the flow of the droplets.
A so-called mixed-flow spray dryer utilizes both parallel and countercurrent flow of solids and gases. Here the drying chamber has a short cylindrical upper section and a long bottom cone. Hot gas is admitted to the drying chamber tangentially near the top. The gas spirals near the walls, then reverses direction and spirals upwardly and leaves through an outlet duct at the top. Feed liquid enters through a spray assembly in the roof of the drying chamber. The liquid evaporates and passes outward into the outer spiral gas stream. The dry solids are swept toward the walls and downwardly to the bottom of the cone where they are removed.
Notwithstanding the type of spray dryer employed (and their continued use in the industry) known spray drying apparatus or systems suffer from a serious disadvantage. Thus in known spray dryers, the dried material often adheres to, and "arches" over, the bottom discharge outlet of the drying chamber which results in the "plugging" and/or stoppage of flow of solids from said discharge outlet.
In this regard and before turning to the details of the present invention, masses of solid particles, especially when the particles are very dry and not sticky, have many of the properties of a fluid. That is, they exert pressure on the side walls of a container, they flow through openings, etc. When an outlet containing free-flowing solids is opened, the material immediately above the opening begins to flow. A central column of solids move downward without disturbing the material at the sides. Eventually lateral flow begins, first from the uppermost layer of solids. A conical depression is formed on the surface of the mass. The solids at, or near, the walls are the last to leave. The material slides laterally into the central column at an angle approximating the angle of the internal friction of the solids. If additional material is added to the mass of solids at the same rate as the material flowing out of the bottom, the solids near the bin walls remain stagnant and do not discharge.
With cohesive or sticky solids, particularly at higher temperatures, it is often even difficult to initiate flow at the outset. Once flow does start however, it again begins in the material directly above the discharge opening. Frequently the column of solids above the discharge outlet moves out as a "plug" leaving a "hole" with nearly vertical sides. Sticky solids, as well as the so-called dry powders adhere strongly to the vertical surfaces and have sufficient shear strength to support a "plug" of considerable diameter.
Further, with very cohesive solids, there is a problem of "arching" which in many cases is strong enough to support the overlaying solid even when the discharge outlet is opened.